126 PLANT STRUCTURES 
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gression of leaves illustrating all three stages may be 
watched in spring in the Arrow-head (Sagittaria 
sagittifolia), The first leaves produced are entirely 
submerged, and conform to the usual ribbon shape 
and delicate texture. Those which follow float on the 
surface. In them the lower part is contracted into a 
flaccid winged petiole, the upper part being expanded 
into an oblong floating blade with a waxy surface to 
keep the leaf dry on the upper side. These in turn 
give way to the characteristic aerial arrow-shaped 
leaves of summer, which approach in character the 
leaves of land plants, and are borne on stout, stiff 
petioles capable of resisting wind and wave. 
Coming now to FLOWERS, it is possible here to refer 
only to a few macroscopic or “naked-eye” characters 
and modifications; the full study of the flower and 
its essential functions being a matter for the labora- 
tory and the high-power microscope, as very minute 
structures are involved. As briefly described in 
Chapter IV., flowers are groups of modified leaves 
arranged mostly very close together at the ends of 
branches, the tip of the shoot being often expanded 
into a receptacle (very well seen in the Composite— 
e.g. Dandelion) for the accommodation of the 
crowded floral leaves. Just as the foliage leaves have 
become modified to carry on to the best advantage 
the process of assimilation, so the different series of 
floral leaves are specially adapted to their several 
functions. The sepals, which compose the calyx, hav- 
ing usually a protective rdle, in most cases enclose the 
young flower with a tough envelope; they usually 
retain their primitive green colour, and take part in 
the process of assimilation. They may drop off as 
